Pyrohydrolytic attack of rare earth fluocarbonate ores



United. States. Patent F 3,375,061 PYROHYDROLYTIC ATTACK OF RARE EARTH FLUOCARBONATEL ORES Robert M. Healy, Warrerrville, Ill., andLamar T. Royer,

Oak Ridge, Tenn., assignorsto American Potash &- Chemical Corporation, Los. Angeles, Calif., a corporation of Delaware No Drawing. Filed Mar. 12, 1964, Ser. No. 351,511 Claims. (CI. 23-15) ABSTRACT OF THE DISCLOSURE A process for recovering substantially all. of the rare earth values from rare earth fluocarbonate ores'by subjecting the ore to pyrohydrolytic attack in which-heated ore is contacted with steam whereby fluorine values in the ore are volatilized and removed therefrom and substantially all of the rare earth values in the ore are converted to acid soluble rare earth oxides which may subsequently be leached from the treated ore. Pyrohydrolytic attack of the ore is generally carried out at tempenatures of between 700 C. and 1300 C. Lower temperatures may be used if the ore is digested with a concentrated mineral acid prior to pyrohydrolytic. attack.

The present invention relates to the treatment of rare earth fluocarbonate minerals. More particularly, the present invention relates to the treatment of rare earth fiuo carbonate ores by pyrohydrolytic procedures.

The treatment of rare'earth bearing ores by acid'leachin-g procedures is known. Heretofore, such procedureshave not been completely satisfactory when applied to rare earth fluocarbonate ores because it has not been pos-- sible to recover more than about 75% by weig ht' of the contained rare earth values, 'basedon the total'weight of rare earth values in the ore. The valueswhich-were not recoverable by acid'leaching techniques were found tobe present as acid-insoluble rare earth-fluorine compounds. Attempts to recover these insoluble values have been-gen erally unsuccessful.

In accordance with'the present invention, there ispro vided a process which-results in-the recovery of substantially all of the rare earth values from fluocarbonate ores, including at least the major portion of the heretofore un-- recoverable rare earth values present as acid-insoluble rare earth-fluorine compounds. I

It will be understood that the term rare earth as used herein includes: those elements of the lanthanide series, having atomic numbers from 57 through 71 inclusive, and the elements yttrium and scandium which maybe present in minor amounts in rare earth fluocarbonate ores.

Broadly, the present invention providesa process for the recovery of rare earthvalues from rare earth fluocarbonate ores, generally bastnasite ores, by subjecting tlie ore, in particulate form, to pyrohydrolytic attack. At'least the major portion of the rare earthvalues in'thepajrticulate ore are converted, by pyrohydrolysis", to acid-soluble rare earth oxides; If desired, pyrohydrolysis of the ore can be preceded by other ore-opening techniques including, for example, pyrolysisto drive off carbon dioxide and convert the rare earth fiuocarbonates to rare earth oxyfluorides; leaching with dilute mineral acids to solubilize the alkaline earth carbonate-principally calcium car-' bonate-portion of the ore; digesting with concentrated mineral acid to dissolve the acid-soluble rare earth values in the ore; or other techniques.

" Patented Mar. 26, 1968 The term pyrohydrolysis used-in this specification,v

The term acid-soluble as used herein, means,.unless otherwise stated, that the identified materialis soluble inv some concentration of either hydrochloric or nitric acid at a temperature between the. boiling and freezing points of the acid mixture under normal atmospheric pressure. Conversely, the term acid-insoluble used hereinmeans, unless otherwise stated, that the identified material is substantially insoluble in any concentration of either nitric or hydrochloric acidat any temperature between the boiling and freezing point of the acid mixture under normal.

atmospheric pressure;

The fluocarbonate ore can be subjected. to. pyrohydrolytic attack as the only recovery procedure to recoversubstantially all. ofthe rare earth'values in the ore as acidsoluble rare earth oxides. If this procedure is followed, the exiting gases from the pyrohydrolytic reaction will contain both carbon dioxide and hydrogen fluoride. Since the carbon dioxide is very volatile and has a very low critical temperature (31.1" (3.), it is generally impractical to condense this-gas; However, if the carbon dioxide is not condensed, itwill carry entrained hydrogen fluoride with it; In orderto simplify the recovery of hydrogen'fluoride,

the pyrohydrolysis preferably is carriedout on rare earth fluocarbonate ore after. it has been treated to remove at least a major portion'of the included carbon dioxidev and:

carbonate.

The rare earth oxides recovered from the pyrohy-- drolysis of. rare earthfluocarbonate ores can be utilized as desired. For example, the rare earth oxides may be dissolved'in an acid solution to separate the rare earth:

values from insoluble gangueand to provide the corresponding rare earth salt. The rareearth bearingsolution; may be separated from-the gangue byfiltration or other convenient means. Rare earth salt can be separatedfrome the solution by conventional procedures, for example-by evaporation. This rare earth salt may be reduced to the rare earth metal. Rare earth metal-may be alloyed withv iron'as a modulating agent. Alternatively, the rare earthsalts maybe converted to pure rare earth oxides which: 1 enjoywide utility as glass polishing compositions. Individual rare earths may be separated from. one another, for example, by ionexchange.

The rare earth fluocarbonate ores to which the present process is applicable include, for example, low grade run of the mine materials as Well as concentrates produced by flotation, acid leaching andother conventional beneficiation processes. Representative examples of ores include those havingthe following approximate weight percentage compositions:

I Concentrate Run-of-Mlne No. 1' No. 2 No. 8

Rare earth oxide 4-11 55 Fluorine 0.4-1. 3 5 5 7 Calcium carbonate 4075 15 1 1 Barium sulfate. 15-50 5 2O 4 Iron oxidepresent 1 1 1 Aluminum oxide present 1 1 1 lica present 2 2 3 Generally, the rare earth fluocarbonate ores to which the present process can be applied include; for example,

those having the following approximate weight percentage compositions:

Percent Fluorine 0. 19 Rare earths 3-80 Alkaline earth metal carbonates 1-75 Silica 0.1-25 Barium sulfate 4-50 Iron oxide 1-5 Aluminum oxide 1-5 Rare earth fiuocarbonate ores of this composition may be found in association with a wide variety of other minerals and valueless gangue materials, some of which may be carried with the fluocarbonate ores into the present process. Such extraneous materials are not generally detrimental to the pyrohydrolysis reaction because the present process can be applied effectively to generally all rare earth fluocarbonate-containing ores.

The rare earth fluocarbonate ores can be subjected to conventional ore-dressing procedures prior to the described pyrohydrolytic attack. Such conventional ore-dressing procedures include, for example, the separation of valueless gangue and other extraneous materials by such techniques as classification, flotation, sedimentation and the like.

The particle size of the rare earth fluocarbonate ore is not critical; however, it is generally possible to increase the rate of reaction by decreasing the particle size, since the surface area, and thus the amount of material exposed to the reaction, increase as the particle size decreases. If the particle size is too large, it is difficult to carry the reaction to completion in a reasonable length of time.

The ore, prior to being subjected to pyrohydrolysis, is preferably treated to reduce its particle size. The particle size of the ore is generally such that it will pass a No. 3 US. Standard screen and it may be as small as 1 micron or less. Preferably the particle size of the ore is such that it passes a 100 mesh (US. Standard) screen. The conventional ore dressing and sizing operations can be accomplished on commercially available apparatus.

Pyrohydrolytic attack of the rare earth fluocarbonate ore can be accomplished either continuously or batchwise in commercially available equipment such as, for example, rotary kilns, furnaces, fluidized bed apparatus, and the like.

The pyhohydrolysis reaction preferably is carried out while the ore is maintained at a temperature between 700 C. and 1300 C., preferably between about 900 C. and 1200 C. At about 700 C. the reaction proceeds rather slowly and may not go to completion, while at around 1300 C., care must be taken to avoid producing a solid sintered mass. The optimum temperature for the convenient pyrohydrolysis of any given ore sample is dependent upon such variable as, for example, the composition of the ore, the particle size of the ore, the treatment of the ore prior to pyrohydrolysis and the like. When the ore has been digested with concentrated acid, the pyrohydrolysis reaction proceeds at a lower temperature generally from about 400 C. to 1300C.

The temperature of pyrohydrolysis should be such that it is suflicient to drive the reaction to substantial completion and render the major proportion of the rare earth values acid soluble in a reasonable period of time. As described more completely hereafter, certain indicators may be relied upon to determine when the reaction is nearly complete. Quantitative analysis of the rare earth values in the solid reactant and reaction product of the pyrohydrolysis reaction will reveal how much of the rare earth value in the solid reactant has been rendered soluble in either nitric or hydrochloric acid by the reaction.

When the major proportion of the rare earth values contained in the ore which were acid-insoluble prior to pyrohydrolysis have been rendered acid-soluble, the reaction is considered to be substantially C mplete. Preferab y,

the pyrohydrolysis reaction is carried to the point at whic at least 85% or more of the acid-insoluble rare earth values have been rendered acid-soluble.

Generally, there is a correlation between the total quantity of hydrogen fluoride evolved during the pyrohydrolysis reaction and the completion of the reaction. In general, the reaction is considered to be substantially complete when 85% or more of the fluorine present in the material fed to the pyrohydrolysis reaction has been removed from the reaction mixture as hydrogen fluoride.

Water can be supplied to the pyrohydrolytic reaction at any temperature desired, even as particles of ice; however,- it is generally desirable to avoid using low temperature water since it may tend to cool the heated bed of particulate ore below the reaction temperature. Preferably, water is introduced to the pyrohydrolysis reaction in the form of super-heated steam of such a temperature that the cooling effect, if any, of the steam on the mass of heated particulate ore is slight.

If desired, heat can be supplied to the mass of particulate ore through the use of steam which is heated to a temperature above that of the particulate ore. This procedure is often advantageous when the reaction is carried out in a fluidized bed.

Water is generally supplied to the pyrohydrolysis reaction in an amount greatly in excess of that required to react completely with the rare earth and fluorine values in the ore. An excess of Water is preferred in order to maintain the maximum rate of reaction.

The time required to complete the pyrohydrolytic reaction, using ore of a given particle size, is largely dependent upon temperature and is determined conveniently by certain observations. Thus, substantial completion of reaction may be determined, for example, by analysis of the solid pyrohydrolysis reaction product and determination of the substantial absence of acid-insoluble rare earth values; or by determination of a substantial decrease in the rate of hydrogen fluoride evolution; or by noting that 85% or more of the fluorine in the solid feed material to the pyro-- hydrolysis reaction has been removed from the reaction as hydrogen fluoride, silicon tetrafluoride and other volatile fluorine compounds.

At least' a part of the carbon dioxide in rare earth fluorocarbonate ores can be removed by pyrolysis prior to pyrohydrolytic treatment. This pyrolysis reaction can be carried out in the substantial absence of water at a temperature above about 500 C. and preferably above about 600 C. or it can be accomplished immediately prior to or concurrently with the pyrohydrolytic reaction in the presence of water. If it is desired to remove any carbon dioxide which may be present in the ore as calcium carbonate, the ore should be heated to a temperature above about 900 C. The fluocarbonate ore is subject to pyrolysis until such time as the evolution of carbon dioxide substantially ceases. The maximum pyrolysis temperature is generally the fusion point of the ore, which is generally above about 1300 C.

If it is not convenient or desirable to remove, by pyrolysis, that portion of the carbon dioxide in the ore which is present as alkaline earth metal carbonate, it can be removed by treating the ore with dilute hydrochloric or nitric acid. Generally, this reaction can be accomplished by admixing the ore with acid until the pH of the mixture declines and remains relatively constant between about 1 and 2. It has been observed that under these condition, substantially all of the carbonate in the alkaline earth metal carbonate is removed as carbon dioxide and very little, if any, of the rare earth values are solubilized. The completion of this reaction is'evidenced by the substantial cessation of gaseous carbon dioxide evolution. This reaction is generally carried out at a temperature between about 20 C. and 50 C.

The pyrohydrolysis reaction may be applied to the insoluble residue from a concentrated acid leach. A concentrated acid leaching step will remove at least a part of the acid-soluble rare earth values which are present in the ore. A concentrated acid leach step can be accomplished using either nitric or hydrochloric acid at elevated temperatures above about 50 C. and up to the boiling point of the mixture. Generally, the more concentrated the acid, the greater will be the amount of rare earth values solubilized in this step. However, since any rare earth values remaining in the ore after this concentrated leach will be recovered in the pyrohydrolysis reaction, it is unnecessary to impose severe enough reaction conditions to solubilize the maximum amount of acid-soluble rare earth in this step. Conveniently, the acid used in this step can range in concentration from 3 normal to 9 normal or even greater.

Typical analytical procedures for the determination of rare earths may be found in Treatise on Analytical Chemistry, Kolthoff and Eving, Part 11, volume 8.

General methods for determination of fluorine in bastnasite are given in Treatise on Analytical Chemistry, Part II, volume 7.

In the specification, claims and following specific examples, all parts and percentages are by weight unless otherwise indicated. The following examples are set forth to further illustrate and not to limit the invention.

EXAMPLE I This example illustrates the optional treatment of rare earth ore with dilute mineral acid to dissolve a portion of the alkaline earth metal carbonate content of the ore before subjecting it to pyrohydrolytic attack.

A bastnasite ore concentrate 90% of which passes a 200 mesh sieve (US. Standard) is found to have the following composition:

Ore components: Weight percent *Ln O +ThO 57.2

T110 r 0.03 ceo 28.6 CaO 11.3 MgO 0.1 sio 1.2 2 3 BaO 3.9 R 0 1.0

Loss on ignition *Ln is a generic symbol used to represent all of the rare earth elements generally, A mixture of rare earth oxides is present here.

**R 0a is a generic symbol used to indicate those compounds which, during systematic analysis, are precipitated from solution by the addition of ammonium hydroxide. and includes iron oxide, aluminum oxide and titanium oxide. Phosphorus pentoxide is co-precipitated with iron oxide and is included here.

A sample of this bastnasite ore composition is admixed with hydrochloric acid solution at ambient temperature. A sufiicient quantity of hydrochloric acid solution is used to bring the pH of the mixture down to a constant value of about 1. The undissolved residue is washed twice by decantation, filtered and dried. This dried leached bastnasite ore concentrate contains 72.9 weight percent rare earth oxides, 4.5 weight percent fluorine and has a loss on ignition of 19.6 weight percent. The composition of the mineral bastnasite can be represented by the formula LnFCO Using CaCO to represent the alkaline earth carbonates in general, the principal reactions taking place during this dilute hydrochloric acid leach can berepresented as follows:

LnFCO -i-dilute HClsubstantially no reaction CaCO +dilute HCl CaC1 +H O-|CO EXAMPLE II This example is illustrative of the pyrohydrolytic treatment of bastnasite ore which has been leached with acid to remove the alkaline earth carbonates, as in Example The experimental apparatus used consists of a nickel tube 33 inches lorig having a 1 inch inside diameter. The first 14 inches of this tube, through which the steam is to be introduced, is enclosed in a prehe-ater which maintains this end of the tube at about 450-480 C. The next 18-inch section of this tube is the main heating furnace in which the temperature is controlled by a resistance heating element capable of maintaining the inside of the nickel tube at any predetermined elevated temperature up to about 1200" C. Steam is introduced into the preheater section where it is heated to a temperature of about 400480 C. before it passes over a Weighed charge of ore in the main heater section. After passing over the ore sample, the steam and gases are passed through a water-cooled condenser and collected for analysis.

The principal reactions taking place during pyrohydrolysis of this leached ore are, first, the pyrolytic reaction:

A LnFCOa LnOF CO:

and, second, the pyrohydrolysis reaction:

A 2LI1OF +H2O Lllzoa 2HF In order to analyze the results of this pyrohydrolytic treatment, the solid product remaining in the main heating furnace is treated with an excess of 6 normal hydrochloric acid. This mixture of solid pyrohydrolytic product and hydrochloric acid solution is heated and agitated for about 30 minutes. After this treatment, the insoluble residue is filtered from the solution, dried, ignited to approximately 900 C. and weighed.

The results of carrying out the procedure of this example under slightly different conditions for several different runs are set forth below.

Run A Starting material g. leached bastnasite. Product of pyrohydrolytic treatment 76.55 g., a 23.5% loss of Weight based on the starting material. Temperature of main heating furnace 1200 C. Time of pyrohydrolytic treatment 3 hrs. Steam volume 3.73.8 mL/min. steam condensate.

Insoluble residue after leaching the solid pyrohydrolytic reaction product 1.7 g.; this residue is equal to 2.2% by weight of the pyrohydrolytic product or 1.7% by weight of the starting material.

weight percent of the calculated available fluorine is collected in the steam condensate and 99.5 weight percent of the calculated available rare earth values, calculated as Ln O are dissolved in the hydrochloric acid solution.

Run B Starting material g. of leached bastnasite. Product of pyrohydrolytic treatment 115.6 g., a 22% loss of weight based on the starting material. Temperature of main heating furnace 1000 C. Time of pyrohydrolytic treatment 4 hrs. Steam volume 3.8-4 ml./min. steam condensate.

70 weight percent of the calculated available fluorine is collected in the steam condensate and 93.2 weight percent of the calculated available rare earth values, calculated as Ln O are recovered in the hydrochloric acid solution.

Run C Starting material 180.6 g. of leached bast- Product of pyrohydrolytic nasite.

treatment 138.2 g., a 23.4% loss of weight based on the starting material.

Temperature of main heating furnace 1100 C. Time of pyrohydrolytic treatment 5 hrs. Steam volume 3.8 ml./min. steam condensate.

Insoluble residue after leaching the solid pyrohydrolytic reaction product 7.44 g., this residue is equal to 5.3% by weight of the product or 4.3% by weight of the starting material.

85 weight percent of thecalculated available fluorine is collected in the steam condensate and 96.8 weight percent of the calculated available rare earth values, calculated as Ln O are recovered in the hydrochloric acid solution.

Run D Starting material 69.8 g. of leached bastnasite. Product of pyrohydrolytic treatment 54.7 g., a 21.5% loss of weight based on the starting material. Temperature of main heating furnace 900 C. Time of pyrohydrolytic treatment 6 hrs.

72 weight percent of the calculated available fluorine is a collected in the steam condensate and 91.4 weight percent of the calculated available rare earth values, calculated as Ln O are recovered in the hydrochloric acid solution.

Run E Starting material 69.5 g. of leached bastnasite. Product of pyrohydrolytic treatment 56.1 g., a 19% loss of weight based on the starting material. Temperature of main heating furnace 800 C. Time of pyrohydrolytic treatment 4 hrs.

61 weight percent of the calculated available fluorine as collected in the steam condensate and 84 weight percent of the calculated available rare earth values, calculated as Ln O are recovered in the hydrochloric acid solution.

EXAMPLE III Prior to pyrohydrolysis the ore used in this example is digested with concentrated mineral acid. The digested bastnasite is pyrohydrolyzed to recover the rare earth values in this material which cannot be solubilizedwith mineral acids.

A 100 gram sample of leached bastnasite ore concentrate prepared by the method of Example I, above, is subjected to attack by 375 ml. of boiling 6 normal hydrochloric acid under reflux for 4 hours. At the end of this 4 hour period, the insoluble residue is separated from the solution and dried. The residue weighs 24.2 grams. Analysis shows it to contain 66% rare earths as oxide and 19.2% fluorine.

The residue is subjected to the pyrohydrolytic treatment, using the same procedure described in Example II, above, at a temperature of 1000 C. and a treatment time of 3 hours. The product of this pyrohydrolytic attack weighs 210 grams indicating a loss in weight of 13.2%.

This pyrohydrolytic product is digested with an excess of boiling 6 N hydrochloric acid. The washed and dried insoluble product from this leaching step is found to weigh 5.2 grams which is 5.2% by weight of the starting material.

Analysis of the solution obtained from the digestion disclose that 14.7 grams or 92% of the rare earth values (calculated as rare earth oxide) present in the insoluble residue prior to pyrohydrolysis are recovered in the leach liquid. Rare earth values remaining in the residue amounted to 1.28 grams.

Determination of fluorine in the steam condensate shows it to contain a total of 4.1 grams which is of the total contained in the leached bastnasite ore concentrate starting material.

EXAMPLE IV This example is illustrative of the pyrohydrolytic attack.

of bastnasite ore which has not been subjected to a preliminary leach with mineral acid and which has the composition shown in Example I, above.

Twenty grams of unleached bastnasite is heated in a tube furnace to 980 C. and 64 ml. of water (as steam) are then passed through the tube at a constant rate in a period of 3 /2 hours. The product weighs 15.56 grams. This product is digested with ml. of 6 normal hydrochloric acid and the insoluble residue from this digestion is dried. The dry insoluble residue weighs 1.0 gram.

EXAMPLE V This example is illustrative of the optional separate pyrolysis treatment to decompose the bastnasite mineral and remove carbon dioxide prior to pyrohydrolysis.

Fifty grams of leached bastnasite, prepared in Example I, above, are placed in a Vycor tube which is connected to a fiowmeter. The tube is heated slowly in a tube furnace. Slow evolution of gas is noted when the temperature of the ore, as shown by a thermocouple placed within the furnace, reaches 580 C. Gas evolution is rapid as the temperature is slowly raised from 600 to 640 C., over a period of one hour. The temperature is slowly increased for an additional hour and at a temperature of 690 C. the evolution of gas ceases.

In subsequent pyrolysis reactions, carried out in the manner described in this example, it is observed that the rate of carbon dioxide evolution can be increased by increasing the pyrolysis temperature. This temperature can be increased up to the sintering point of the ore.

In the subsequent pyrohydrolysis of this pyrolized material, the reaction is initiated at a temperature of about 700 C. and proceeds at a commercially practical rate at 900 C.

EXAMPLE VI This example is illustrative of the pyrohydrolytic attack on rare earth fiuocanbonate material which has been subjected to concentrated acid digestion.

Thirty grams of insoluble residue obtained by the concentrated acid digestion procedure of Example III, above, is subjected to the action of water vapor at a temperature of 750 C. for a period of five hours. Water is introduced continuously at a rate of 18 grams per hour. The first fractions of condensate collected contain 25 to 33 percent hydrogen fluoride by Weight. After 75 minutes, the concentration of fluorine has dropped to 5.2 percent; after 175 minutes, to 2.4 percent; after 240 minutes, to 0.7 percent and after 320 minutes, the fluorine concentration is down to 0.3 percent. At this point, the treatment is terminated.

The product of pyrohydrolysis is digested with 6 N hydrochloric acid. The solution is separated from the small insoluble residue and analyzed. Eighty-seven percent of the rare earth values originally present in the sample prior to pyrohydrolysis is found to be present in the solution as rare earth chloride.

This reaction is repeated to determine the optimum temperature range for carrying out the pyrohydrolysis of rare earth fluocarbonate material which has subjected to a concentrated acid digestion. It is found that pyrohydrolysis begins at 400 C., is fairly rapid at 500 C. and proceeds at a commercially practical rate at 600 C. and higher.

As will be understood by those skilled in the art, what has been described is the preferred embodiment of the invention; however, many modifications, changes and substitutions can be made therein Without departing from the scope and the spirit of the following claims.

What is claimed is:

1. A process of treating rare earth fluocarbonate ore which comprises pyrohydrolytically attacking said ore by contacting said ore with steam while the ore is at a temperature of between about 700 and 1300 C. for a period of time suflicient to volatilize a major portion of the fluorine values from said ore and leave a solid product in which a major portion of the rare earth values in said ore have been converted to rare earth oxides which are soluble in inorganic mineral acid.

2. The process as defined in claim 1 in which said ore is contacted with steam which is at a temperature suflicient to heat said ore to a temperature of between about 700 C. and 1300 C.

3. The process as defined in claim 1 in which the ore is heated to a temperature of between about 700 and 1300 C. and is then contacted with steam.

4. The process as defined in claim 1 in which said solid product is admixed with an acid selected from the group consisting of hydrochloric acid and nitric acid to dissolve said rare earth oxides in said acid and produce a liquid mixture containing dissolved rare earth values.

5. The process as defined in claim 1 in which said ore is subjected to pyrolysis prior to said pyrohydrolytic attack by heating said ore to a temperature above about 500 C. in the substantial absence of water to remove at least a portion of the carbon dioxide from said ore.

6. The process as defined in claim 1 in which said ore is admixed with a dilute inorganic mineral acid prior to said pyrohydrolytic attack to remove at least a portion of the carbonate values contained in said ore.

7. The process as defined in claim 6 in which said acid is selected from the group consisting of dilute hydrochloric and dilute nitric acid and said acid is admixed with the ore in an amount sufficient to reduce the pH of said mixture to a value between 1 and 2 to remove substantially all of the carbonate values from the ore prior to said pyrohydrolytic attack.

8. A process for treating rare earth fluocarbonate ore which comprises admixing said ore with a concentrated inorganic acid to form a mixture of solubilized rare earth values and an insoluble residue containing acid insoluble rare earth values, separating said acid insoluble residue from said solubilized rare earth values and subjecting said insoluble residue to pyrohydrolytic attack by contacting said residue with steam at a temperature of between about 400 and 1300 C. for a period of time suificient to volatilize a major portion of the fluorine values from said residue and leave a solid product in which a major portion of the rare earth values have been converted to rare earth oxides which are soluble in inorganic mineral acid.

9. The process as defined in claim 8 in which said residue is contacted with steam which is at a temperature suificient to heat said residue to a temperature of between about 400 and 1300 C.

10. The process as defined in claim 8 in which said residue is heated to a temperature of between about 400 and 1300 C. and is then contacted with steam.

References Cited UNITED STATES PATENTS 1,129,029 2/1915 Vogt 23-16 X 2,735,747 2/1956 Kasey 23-16 FOREIGN PATENTS 222,950 7/1959 Australia.

OSCAR R. VERTIZ, Primary Examiner.

H. T. CARTER, Assistant Examiner. 

